Final answer:
The principal diagnosis would be 645.11 Post-term pregnancy, delivered, with or without mention of antepartum condition.
Step-by-step explanation:
The principal diagnosis in this case would be 645.11 Post-term pregnancy, delivered, with or without mention of antepartum condition. This code is used for a patient who has delivered a baby at or after 42 weeks of gestation. The patient in this scenario is admitted in labor at 43 weeks, so the post-term pregnancy code is the most appropriate.